U.S. Pat. No. 5,038,325, entitled "High Efficiency Charge Pump Circuit" describes one charge pump and is herein incorporated by reference.
In order to pass a full charge to a DRAM memory cell through an n-channel access transistor, it is necessary to drive the gate of the n-channel access transistor to a voltage greater than the voltage used to charge the storage capacitor. Modem dynamic random access memories (DRAMs) use charge pumps to generate this higher potential.
A typical external supply potential is referred to as V.sub.CCX. V.sub.CCX is often regulated. The regulated potential is referred to as V.sub.CC. In many applications V.sub.CCX is equal to 5 volts and V.sub.CC is regulated to 3.3 volts. A potential generated in a charge pump is generally referred to as V.sub.CCP. In typical cases V.sub.CCP is two volts greater than the DRAM's internal regulated voltage, or two volts above V.sub.CCX for DRAM's that do not use a regulated V.sub.CC.
Previous charge pump circuits have had difficulty operating with a V.sub.CC below three volts. There have been attempts to design circuits to overcome this problem.
In one attempt to increase the efficiency of a charge pump having a regulated supply potential, a level translator circuit was added between the logic of the pump, and the pump capacitors. The level translator allowed the circuit to draw current from the external power source V.sub.CCX rather than the regulated source V.sub.CC. With this circuit a potential of (2V.sub.CCX -V.sub.T) could be passed through a pump n-channel transistor as V.sub.CCP, where V.sub.T is the threshold voltage of the pump n-channel transistor. Thus for a V.sub.CCX of 4 volts, the V.sub.CCP is equal to 7 volts for a V.sub.T of 1 volt. This is sufficient for transferring a full V.sub.CC into a memory location of a DRAM. In early regulated devices, V.sub.CC may range from 3.5 to 4 volts. However, there is currently a trend toward lower levels of V.sub.CCX. Current specification require V.sub.CCX of 2.7 to 3.6 volts, and the trend is toward even lower V.sub.CCX levels. In systems with low C.sub.CCX, there is no longer a need for a regulated V.sub.CC. Instead, V.sub.CCX is used throughout the circuit. V.sub.CCP is still required to pass a full V.sub.CCX in to the DRAM memory cell. As V.sub.CCX is reduced, the efficiency of the V.sub.CCP pump is also reduced since the charge available for transfer to V.sub.CCP is proportional to V.sub.CCX times the capacitance of the pump capacitor. Thus as V.sub.CCX is reduced, the pump eventually fails.
In another attempt 2V.sub.CCX can be passed through as V.sub.CCP in a two stage pump by using one pump to generate the entire supply voltage for a second pump. This approach is inefficient for a low level supply potential and is impractical since all the charge eventually is generated from the first pump. In this configuration a very large first stage pump is required to provide the required supply potential for the second stage pump.
Thus a need exits to provide a pumped potential at the gate of the access transistor such that the full charge on the memory storage capacitor is passed through the access transistor for devices with low V.sub.ccX.